Reduce branching (or batch reorganization) overheads

[Joongi Kim]

In the EuroSys 2015 paper, we demonstrated a simple branch prediction technique.
However, after Packet/PacketBatch refactoring (#1), the current branch prediction technique does not perform well.
In this issue, we investigate why this happens and find another way to improve branching performance.

Anticipated overhead sources:

1. Allocation of new batch objects from the memory pool
2. Copying packet pointers to the new batches
3. Handling packet masks in the original batch (if reused)

Test scenarios:

• single branch
• balanced tree (both majority/minority outputs have the same next branches)
• majority-skewed tree (next branch connected to the majority output of the current branch; minority outputs are short-circuited to L2Forward)
• minority-skewed tree (next branch connected to the minority output of the current branch; majority outputs are short-circuited to L2Forward)
• some processing elements after branch - to highlight the 3rd overhead